Mayor John DeStefano, the longest-serving chief executive in city history, will not run for reelection after 20 years in office, a city official said Tuesday. Jacqueline James, chairwoman of the Democratic Town Committee and president pro tem of the Board of Aldermen, said DeStefano told her Monday he will not seek an 11th two-year term. The 57-year-old Democrat championed immigration-friendly policies. DeStefano was the Democratic nominee for governor in 2006, but lost to Republican M. Jodi Rell.